OBLIQUITIES OF HOT JUPITER HOST STARS: EVIDENCE FOR TIDAL INTERACTIONS AND PRIMORDIAL MISALIGNMENTS

TLDR

Low‑obliquity systems have short tidal timescales, while high‑obliquity systems have long timescales. The authors examine the hypothesis that hot‑Jupiter obliquities arise from dynamical interactions and outline needed observations. They base their evidence on 14 new Rossiter–McLaughlin measurements of various hot‑Jupiter systems and a review of prior data. The results indicate that hot‑Jupiter host stars initially had random obliquities, with low obliquities produced by tidal realignment, suggesting that hot Jupiters form through dynamical processes such as planet–planet scattering or Kozai cycles rather than disk migration.

Abstract

We provide evidence that the obliquities of stars with close-in giant planets were initially nearly random, and that the low obliquities that are often observed are a consequence of star–planet tidal interactions. The evidence is based on 14 new measurements of the Rossiter–McLaughlin effect (for the systems HAT-P-6, HAT-P-7, HAT-P-16, HAT-P-24, HAT-P-32, HAT-P-34, WASP-12, WASP-16, WASP-18, WASP-19, WASP-26, WASP-31, Gl 436, and Kepler-8), as well as a critical review of previous observations. The low-obliquity (well-aligned) systems are those for which the expected tidal timescale is short, and likewise the high-obliquity (misaligned and retrograde) systems are those for which the expected timescale is long. At face value, this finding indicates that the origin of hot Jupiters involves dynamical interactions like planet–planet interactions or the Kozai effect that tilt their orbits rather than inspiraling due to interaction with a protoplanetary disk. We discuss the status of this hypothesis and the observations that are needed for a more definitive conclusion.